Thursday, 18 October 2012

Current Theorems - Am I on Track?

Perhaps my previous calculations of power consumption are too simplistic.

In researching green options, I discovered on some forums the use of a 5 port USB powered hub to deliver power to the RPi, and its peripherals, with the RPi delivering communications and data back to the hub.  I had been wondering about the concept of using one hub to power multiple RPis - would it be more efficient and greener?

Both the RPi and the hub run from their own transformers.  The RPi transformer has an input of 240V at 50-60Hz, drawing 150mA, and delivers a DC output of 1A 5VA.  To my limited knowledge of the calculations involved, this means that the input power is: 240 x 0.15 = 36 Watts, and the output is: 5 x 1 = 5 Watts.  This would imply that 31 Watts is lost to the resistance and impedance of the unit, and generates heat in the transformer.

When I look at the input and output of the USB hub's transformer, the input is: 240V at 50-60Hz, drawing 1A, therefore 240 Watts.  The output is 5V 1.5 A, therefore 7.5 Watts.

This would imply that whilst the powered hub supplied 50% more power, it draws 660% more power from the mains, and hence would not be a greener more efficient supply.

However, I know that my knowledge in this area is presently limited - am I using the right calculations and making the right deductions? Should I be calculating and comparing the power consumption of the transformers from their input or output values? Are these constant loads, or the maximum loads of each unit?

Anyone able to help?